How to build clang modules to work with Swift/C++ interop

You can begin here: https://www.swift.org/documentation/cxx-interop/

Already followed all that and it doesn’t work.

Are you targeting Swift package manager? Or Xcode?

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

I have C++ Xcode projects and Swift projects. These only work right now with a c-style bridging header. Apple doesn’t have any sample apps that build clang modules sinice they were released many years ago. Watched the WWDC on explicit modules, but then read these don’t work with Swift interop.

Have a modulemap file, -fmodules defined, but Swift still can’t see the C++ sources. one of the strengths of Apple development was ease of interop between ObjC++ and C++. Have had to wait until Swift 5/6 for something not under a C-wrapper. My code is on Swift 5 if that matters. But switching to Swift 6 is a concurrency warning flood.

Also when I have Swift 5 set in Xcode 16, is that 5.xx (f.e. 5.9 has interop) or 5.0?

Also when I have Swift 5 set in Xcode 16, is that 5.xx (f.e. 5.9 has interop) or 5.0?

That’d be Swift 6 (-:

There’s a difference between the compiler version and the language mode. Xcode 16 only includes the Swift 6 compiler. The Swift Language Version build setting controls the language mode.

And, yes, that’s confusing, and there’s an active effort to improve how we talk about this stuff. See SE-0441 Formalize ‘language mode’ terminology.

AFAIK there’s no difference between the Swift 5 and 6 language modes when it comes to C++ interop. The mode increment is all about Swift concurrency enforcement, and it’s fine to ignore that for the moment.

C++ interop is still cutting edge stuff, so it’s not as smooth as it should be. However, you should be able to get the basics working.

I have C++ Xcode projects and Swift projects.

I’m presuming that last bit was meant to be “Swift Xcode projects”.

So how do you plan to glue them together? For example, do you want to create a framework in C++ and use it in Swift? Or vice versa? Or do you want to have a single target that includes both Swift and C++?

For context, I wanna play around with this to make sure that I understand it myself. Hopefully I’ll then be able to provide some better guidance. However, it’d best if my exploration was aligned with your general goal. So, I’d like to clarify your goal before I start playing (-:

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

I’m presuming that last bit was meant to be “Swift Xcode projects”.

Yes, it's all Xcode. In a workspace, with separate swift and C++ code. The C++ is vector math and a texture encoder/decoder. The Swift is visionOS stuff, a build profiler, etc. Mostly forced into Swift use by Swift-only frameworks like SwiftUI and RealityKit.

I'd like to move code off Swift and onto C++. This would primarily be Swift calling C++, and avoiding writing a bridging header.

So I think I have modules for C++ building, but then get errors trying to import them into Swift.

c++

module vectormath234 {
    // All headers are pulled in by this.
    requires cplusplus20
    
    // This defaults to namespace simdk
    header "vectormath234.h"
}

Swift

import vectormath234

module.modulemap:1:8 Module 'vectormath234' requires feature 'cplusplus20'

MathHelpers.swift:6:8 Could not build Objective-C module 'vectormath234'

I have -fmodules and -fcxx-modules set on the compile line.

Thanks Quinn if you have time to address this. I really appreciate your great dev support.

Also wondering how to interop one C++ lib with another via clang modules. This document says there is no syntax for that, and that only ObjC has the "@import module" syntax. Given that it's over 4 years since C++20, and still no C++ 20 module support, I've resigned to just use clang modules. But it's unclear what they do or don't support. The docs below state that C++ is possible, and just need the "requires cplusplus" designator, but see above errrors.

https://prereleases.llvm.org/18.1.0/rc3/tools/clang/docs/Modules.html

I also found this setting in Xcode, so set that for C++/Obj-C++ itnerop. That seems to change the errors that I get about ObjC. Will have more time to revisit this later.

SWIFT_OBJC_INTEROP_MODE = objcxx